In many areas of bearing design, and particularly in the field of high pressure turbo machinery, the design of the rotor-bearing system is of great importance to the performance of the machine. Certain previous designs have provided rolling element bearings. However, at high speeds and high pressures, the load capacities and stiffness limits of rolling element bearings are exceeded and performance and life comprises are made. An example of this situation is the space shuttle main engine (SSME) high pressure fuel pump. In this device, ball-bearings were placed at the outboard ends of the shaft to accommodate bearing speed (DN) limits, which lead to super critical (i.e., operating above critical speed) rotor design. In turn, the super critical rotor design resulted in an unstable rotor which was subject to destructive and unpreventable subsynchronous whirl and to large radial deflections.
Because rolling elements bearings are subject to speed limitations, load capacity limitations, and short life, design of high performance rotating devices such as high pressure turbo pumps has included work on fluid film bearings. Fluid film bearing design has centered around two categories of devices, namely those that rely on the dynamics of the fluid, such as those created from the motion of the rotor, to create the desired bearing film (generally referred to as "hydrodynamic" bearings) and those that provide a flow of fluid, generally pressurized fluid from an external source, to create the bearing, with withdrawal of the provided fluid (referred to as "hydrostatic" bearings) often, with recycling of the withdrawn fluid. Although hydrodynamic bearings are useful in many applications, certain disadvantages are associated with hydrodynamic bearings, particularly in the context of high pressure turbo machinery. In applications where a fluid has a low viscosity (such as cryogenic fluids like liquid hydrogen, liquid oxygen) the pressure generated by hydrodynamic fluid film bearings is very low (typically having values such as 50 P.S.I., 345 kPa).
The present invention is directed to a fluid film hydrostatic bearing, rather than hydrodynamic bearings. Previous hydrostatic bearings have been associated with a number of problems. In previous hydrostatic bearings, particularly those used in connection with low viscosity fluids, designs intended to produce high direct stiffness (e.g., 3.times.10.sup.6 lb./in.) have also produced undesirably high cross-coupled stiffness (such as about 3.times.10.sup.5 lb./in. or more). Stiffness has particular importance in supercritical devices.
Requirements for bearing performance in connection with subcritical rotor speeds are very different from those used for super critical rotor speeds. Turbo pumps such as those for pumping liquid hydrogen, require high speed, and often super critical rotor speeds to generate the desired pressure. In subcritical designs, the bearing stiffness can be designed very high and can be designed to provide approximately 20% critical speed margin above operating speed. A super critical design, however, typically requires operating between the second and third critical speed and still retaining some margin. As a result, the stiffness requirement has a small range with both a minimum and a maximum bearing stiffness.
Furthermore, the damping characteristics of previous hydrostatic bearings used in connection with low viscosity fluids has been limited. When high pressure turbo machinery used in connection with low viscosity fluids is designed to achieve a small clearances for the hydrostatic bearings, the scale of shaft deflections and tolerances are undesirably large relative to the operating clearances of the bearings. At the high pressures and speeds at which hydrogen turbo pumps operate, deflection of the shaft and housing have a magnitude which is significant relative to the hydrostatic bearing clearances.